1. Field of the Invention
This invention relates to a hydraulic control system for an automatic transmission.
2. Description of the Prior Art
An automatic transmission generally comprises a transmission gear mechanism, and a hydraulic control system including a hydraulic control circuit which controls the hydraulic pressure fed to or discharged from friction coupling members of the transmission gear mechanism, thereby changing the power transmission path of the transmission gear mechanism and causing the automatic transmission to shift, and a control means which controls the hydraulic control circuit according to the running condition of the vehicle so that the transmission automatically shifts to a desired gear speed.
As disclosed, for instance, in Japanese Unexamined Patent Publication No. 62(1987)-13848, there has been put into practice an automatic transmission in which a desired transmission control mode can be manually selected from a plurality of transmission control modes. The transmission control modes typically includes a power mode, an economy mode and a holding mode. In the power mode and the economy mode, the gear-shifting is effected on the basis of predetermined gear-shifting characteristic curves. The gear-shifting characteristic curves for the power mode are determined so that a better output performance can be obtained and those for the economy mode are determined so that a better fuel economy can be obtained.
When the holding mode is selected, the gear-speed is fixed to one set by the driver, generally a higher one, e.g., third in the case of D range, second in the case of 2 range and 1 range.
Further, in the automatic transmission disclosed in U.S. Pat. No. 4,450,941, the line pressure is reduced when the transmission is in a higher gear-speed so that the driving torque of the hydraulic pump is reduced since the transmitting torque of the transmission is small and accordingly the line pressure need not be high when the transmission is in a higher gear-speed. Such a control is generally called "cutback control".
When the cutback control is effected when the vehicle is started with the holding mode selected, the starting performance is deteriorated. That is, in the holding mode, the transmission is held in the higher gear-speed (e.g., second in D range) when the vehicle is started and accordingly, the line pressure is automatically reduced. In this state, the starting performance can be deteriorated due to slips at various parts of the transmission.